chore(client): fix logger property type

Author: stainless-app[bot]

src/client.ts

@@ -222,7 +222,7 @@ export class Gradient {
   baseURL: string;
   maxRetries: number;
   timeout: number;
-  logger: Logger | undefined;
+  logger: Logger;
   logLevel: LogLevel | undefined;
   fetchOptions: MergedRequestInit | undefined;